1. Field of the Invention
The present invention relates to an ignition device equipped with a DC-DC converter, more particularly, to an ignition device with the capability of controlling the operating time of a DC-DC converter.
2. Description of the Prior Art
An example of conventional ignition devices is shown in FIG. 1. In this ignition device 2, the output of a crank angle sensor 4, which generater power at about the ignition time in synchronism with the engine rotation, is input to a transistor ignition circuit 6. The ignition circuit 6, in synchronism with the engine rotation intermittently releases electric current I.sub.1 to the primary windings L.sub.1 of an ignition coil 8, generating a high tension pulse V.sub.p of 15 kV in the secondary windings. The high tension pulse V.sub.p is input to a distributor 10 whose center electrode r rotates synchronously with the engine rotation, whereby through a plurality of output terminals of the distributor and via high tension cords 12a-12d it is impressed to prescribed ignition plugs 14a-14d in prescribed order, to generate discharges in the ignition plugs 14a-14d. The output of the DC-DC converter which boosts the 12 V of a battery 16 to a high voltage of -2 kV is continuously supplied to the low voltage side of the secondary windings L.sub.2 of the ignition coil 8. Therefore, when the ignition plug 14a installed on the first cylinder starts to discharge due to the high tension pulse V.sub.p, and thus turns the ignition plug 14a electrically conductive, the high voltage of -2 kV is supplied to the ignition plug 14a via the secondary windings L.sub.2 through the route of the distributor 10, the high tension cord 12a, and the ignition plug 14a. In this way, by supplying a high voltage of -2 kV from the DC-DC converter to the ignition plugs 14a-14d which started to discharge under the high tension pulse V.sub.p, and finding themselves in the electrically conductive state, the duration of discharges at the ignition plugs can be maintained at more than twice the discharge duration (usually 1-2 ms) due to electromagnetic induction energy supplied by the ignition coil 8. Namely, by raising the discharge energy to a high level (in excess of 100 mJ) which is over twice the corresponding energy for an ignition device not equipped with the DC-DC converter, an improvement in the fuel consumption is achieved through stable and sure ignition and combustion for engines, even under unfavorable combustion conditions.
However, ignition devices equipped with the conventional DC-DC converter are constructed in such a way as to have the DC-DC converter operational all the time so that the DC-DC converter will be operating even prior to the start of ignition and until the latter period of combustion where the combustion improvement measures have no effect, depending upon the speed of engine rotation, and as a result, power is consumed wastefully, aggravating in this respect the fuel consumption. In addition, as a consequence of unnecessarily prolonged discharge times, a deterioration in the durability of the device results due to increased consumption of ignition plug electrodes and to increased heating of the DC-DC converter, ignition coil, and so forth. Moreover, because of the high voltages impressed on the ignition plugs since the time prior to the ignition start, there remain troubles such as the engine malfunctioning due to occurrence of discharges at irregular times other than the prescribed ignition times.